Bleeding into the bowel is currently the best early indicator of bowel cancer (also know as colorectal cancer). Testing for symptoms of bleeding into the bowel is usually achieved by screening stools for the presence of blood. This test is often referred to as fecal occult blood testing (referred to as “FOBT”).
Chemical tests are most widely used for FOBT. These tests typically require stool to be applied to paper impregnated with a chromogen indicator, such as guaiac or 3,3′,5,5′-tetramethylbenzidine (TMB), which changes color on oxidation. When developer solution is added to the paper, a blue color develops with a positive result. Guaiac tests have the advantage of being inexpensive and easy to perform, but are less accurate (not specific for human blood) and less sensitive than desirable. Nevertheless, several international studies have shown that screening patients with these tests can save lives through the early detection of pre-cancerous and cancerous lesions. The commonly used guaiac tests detect the heme of hemoglobin, and as this is relatively resistant to breakdown in the small intestine, these tests may detect bleeding anywhere within the intestinal tract. For colorectal cancer screening this may be a disadvantage as these tumors are confined to the large intestine.
Recently, more sensitive and specific immunological tests (e.g. immunochromatographic tests) have been developed that have the potential to improve the accuracy of detecting blood in screening for colorectal cancer. These tests typically detect the globin protein of hemoglobin, a protein that does not survive passage through the upper gastrointestinal tract. A positive immunological test therefore indicates lower gastrointestinal bleeding. In common with all immunologically based tests, however, these tests are subject to a “prozone” or “high dose hook” effect, where at high levels of analyte, the test may be inhibited to the extent that heavy bleeding may be missed.
Heme from hemoglobin has a pseudoperoxidase activity that catalyses the breakdown of peroxide substrates and the release of oxygen. The released oxygen may be detected by suitable chromogenic indicators such as guaiac and tetramethylbenzidine (TMB) which change color on oxidation. Fecal Occult Blood Tests (FOBTs) detect intestinal bleeding by use of this reaction to detect heme from the hemoglobin of red blood cells, and a variety of formats for such tests are known in the art (see, for example, U.S. Pat. Nos. 3,996,007; 4,225,557; 4,789,629; 5,064,766; 5,100,619; 5,106,582; 5,171,528; 5,171,529 and 5,182,191). Typically, FOBTs involve smearing a stool sample on guaiac-impregnated paper and adding a developer solution containing peroxide. If heme is present, a blue color develops on or around the stool specimen. The disadvantages of these tests include:                the stool sample may also contain peroxidases or pseudoperoxidases from ingested foods and these may cause a (false) positive reaction in the absence of human blood from the intestinal tract;        heme from ingested meat may also cause a false positive reaction;        the blue color developed with a positive test must be read against a dark background of stool, so that at lower heme concentrations the result may be equivocal;        with a positive result, color diffuses away from the stool sample, becomes weaker in intensity, and may fade out (the transitory nature of the color change may make also make interpretation of the test result difficult or unreliable);        the developer solution, containing peroxide and other reagents, can interfere with immunochemical tests that may otherwise be used in conjunction with this test for differentiation between upper and lower gastrointestinal bleeding (see, for example, International Patent Publication WO 00/29852, Enterix Inc., combining a chromogen test to detect any intestinal bleeding and an immunochemical test to detect lower intestinal bleeding only).        
FOBTs have also been described that have a peroxide reagent such as cumene hydroperoxide dried in a paper matrix (see, for example, Lam, U.S. Pat. No. 4,071,318). In this case, the test paper can be added directly to water and will develop color if heme is present in the water. These FOBTs are typically added to a toilet bowl containing a stool after a bowel movement in order to detect blood released from the stool into the water. The disadvantages of these tests include:                blood on, or in, the stool may not diffuse into the water in sufficient concentration to allow detection;        the test must be read against a background of stool and toilet paper, making interpretation difficult;        the tests may also be subject to interference from dietary heme or peroxidases if there is direct contact between the stool and test paper;        the undeveloped test papers must be stored after manufacture in desiccated conditions to prevent breakdown of the peroxide reagent and development of color in the test paper.        